C.O.R.N. Newsletter 2010-20

Dates Covered: 
July 7, 2010 - July 13, 2010
Editor: 
Greg LaBarge
Soybean Disease Early July Status Frogeye Leaf Spot

Soybean Disease Early July Status

Summary of some key findings in the US and Ohio, we will start first with soybean rust.

  1. More locations were found positive for soybean rust in Florida this week.  These are now posted on the IPM PIPE public website (http://sbr.ipmpipe.org/cgi-bin/sbr/public.cgi).  These are sites have been positive in the past, due to the very low inoculum that survived over the winter, they are now at detectable levels.  They have had considerable amount of rain over the past few weeks, so expect to see more counties turn red over the next 2 weeks in the southern part of the US.  The heat may delay it a bit, but overall, these late finds still indicate that the risk for rust to impact Ohio soybeans is very low.
  2. White mold, soybeans that are flowering this week, have limited canopy development are at low risk for this disease.  Many of the soybeans that were planted late, have had a difficult time putting canopy on and this week of hot weather will knock out most of the sclerotia.  The heavy clays are baking those sclerotia right back into the ground.  Soybeans that canopied early, flowered last week during the cool nights will be the fields to monitor for white mold come late August.
  3. Soybean Cyst Nematode – haven’t really spoken about this one in a while, but there are some indications in some of our fields with very high populations, that SCN is taking a toll already this year.  Plants were stunted and the cysts, which are white in color when they first form, were easily seen on the roots.  If you have some stunting of plants in areas that were not flooded, take a shovel and dig some plants up.  Talk to yourself for a bit, while you shake the soil off and let the soil dry that is clinging to the roots.  Then take a close look at the roots, first locate the nodules, round to irregular in shape, which are fixing nitrogen and they should be pink on the inside.  Then look a bit closer for bright white, should be a bit shiny, but about the size of the pin head on the roots.  If there are some, this is a field to target for soil sampling in the fall.
  4. Frogeye leaf spot, now is the time to begin to search of this soybean foliar pathogen on susceptible varieties.  So far no reports in Ohio.  When scouting fields where susceptible varieties are grown, target those that are no-till as this is a residue borne disease, along with those that have a good canopy.  The silver to gray lesions surrounded by purple to red ring will be very few and spaced far apart.  On the underside, it should look like tiny whiskers of the lesion, those are the spores.  Focus on areas of the field where the dew sits longer and the canopy is thicker.  We have gotten a response to fungicides when frogeye lesions were present at the rate of 1 lesion per 25 feet of row, on a highly susceptible variety.  If no lesions were present at the R2 phase, there was no response to fungicides.

 Frogeye Leaf Front ViewFront View of Frogeye Infected Leaf

Frogeye Leaf Backside of LeafBackside of Frogeye Infected Leaf

 

Key Steps in Corn PollinationPollinating Corn Courtesy Bob Nielsen, Purdue University

Key Steps in Corn Pollination

During the past week, tassels began appearing in corn fields that were planted in late April and early May. Corn planted in late May and early-mid June will probably not be tasseling until early to mid August. There is usually some variability in pollination within corn fields and the pollen shed period extends from one to two weeks. This year we can expect even greater variability because saturated soil conditions and loss of nitrogen in poorly drained and compacted areas have inhibited corn growth. The pollination period, the flowering stage in corn, is the most critical period in the development of a corn plant from the standpoint of grain yield determination.

Stress conditions such as drought have the greatest impact on yield potential during the reproductive stage. Past research indicates that four days of stress (i.e. corn wilted for four consecutive days) at the 12th-14th leaf stage has the potential of reducing yields by 5 to 10 percent. The potential for yield losses to soil moisture deficits increases dramatically when plants begin to flower. During tassel emergence, four days of moisture stress has the potential to reduce yields 10 to 25%. Silk emergence is the most critical period in terms of moisture use by the plant. During this stage, leaves and tassels are fully emerged and the cobs and silks are growing rapidly. Four days of moisture stress during silk emergence has the potential to reduce yields 40 to 50%. However, the stress conditions we are alluding to over these “four day periods” are severe and involve extensive leaf rolling (characterized by plants with “pineapple” like leaves) throughout much of the day. Also keep in mind that the corn plant is more vulnerable to hail injury during the period from tassel emergence (VT) to silking (R1) than during any other period because the tassel and all the leaves are completely exposed. Complete defoliation of the plant at VT usually results in 100% yield loss.

The following are some key steps in the corn pollination process.

  • Pollen shed usually begins two to three days prior to silk emergence and continues for five to eight days with peak shed on the third day. Under very dry conditions, silk emergence may be delayed, and such “asynchronization” of pollen shed and silking may result in poor kernel set and reduced grain yields. However, in some years under favorable growing condition, silks may actually emerge before tassels fully emerge and pollen shed starts in certain hybrids.  On a typical midsummer day, peak pollen shed occurs in the morning between 9:00 and 11:00 a.m. followed by a second round of pollen shed late in the afternoon. Pollen may be shed before the tassel fully emerges (“stretches out"). Pollen shed usually begins in the middle of the central spike of the tassel and spreads out later over the whole tassel with the lower branches last to shed pollen.
  • Pollen grains are borne in anthers, each of which contains a large number of pollen grains. The anthers open and the pollen grains pour out in early to mid morning after dew has dried off the tassels. Pollen is light and is often carried considerable distances by the wind.
  • Pollen shed is not a continuous process. It stops when the tassel is too wet or too dry and begins again when temperature conditions are favorable. Pollen stands little chance of being washed off the silks during a rainstorm as little to none is shed when the tassel is wet. Also, silks are covered with fine, sticky hairs, which serve to catch and anchor pollen grains.
  • Under favorable conditions, pollen grain remains viable for only 18 to 24 hours. However, the pollen grain starts growth of the pollen tube down the silk channel within minutes of coming in contact with a silk and the pollen tube grows the length of the silk and enters the female flower (ovule) in 12 to 28 hours.
  • A well-developed ear shoot should have 750 to 1,000 ovules (potential kernels) each producing a silk. The silks from near the base of the ear emerge first and those from the tip appear last. Under good conditions, all silks will emerge and be ready for pollination within 3 to 5 days and this usually provides adequate time for all silks to be pollinated before pollen shed ceases.
  • Pollen of a given plant rarely fertilizes all the silks of the same plant. Under field conditions 97% or more of the kernels produced by each plant may be pollinated by other plants in the field. The amount of pollen is rarely a cause of poor kernel set. Each tassel contains from 2 to 5 million pollen grains, which translates to 2,000 to 5,000 pollen grains produced for each silk of the ear shoot. Shortages of pollen are usually only a problem under conditions of extreme heat and drought. As noted above, poor kernel set is more often associated with poor timing of pollen shed with silk emergence – with silks emerging after pollen shed (poor “nick”). However, hybrids rarely seldom exhibit this problem unless they experience extreme drought stress.

For more information on pollination in corn, check out the following references. Dr. Bob Nielsen, the corn extension specialist at Purdue University, has written excellent articles (see below) on pollen shed and silk emergence that include some great images of the pollination process.

Nielsen, R.L. 2010. Tassel Emergence and Pollen Shed. Corny News Network, Purdue Univ. [On-Line]. Available at http://www.agry.purdue.edu/ext/corn/news/timeless/Tassels.html (URL verified 7-6-10).

Nielsen, RL (Bob). 2010. Silk Emergence. Corny News Network, Purdue Univ. [On-Line]. Available at http://www.kingcorn.org/news/timeless/Silks.html Available at http://www.agry.purdue.edu/ext/corn/news/timeless/silks.html (URL verified 7-6-10).

Ritchie, S.W., J.J. Hanway, and G.O. Benson. 2008. How a Corn Plant Develops. Iowa State Univ. Sp. Rpt. No. 48. [On-Line]. Available at http://www.extension.iastate.edu/hancock/info/corn.htm [URL verified  7-6-10].

New Corn, Soybean, Wheat, and Alfalfa Field Guide for 2010 Cover and Contents of Corn, Soybean, Wheat, and Alfalfa Field Guide

New Corn, Soybean, Wheat, and Alfalfa Field Guide for 2010

A new version of Bulletin 827 Corn, Soybean, Wheat, and Alfalfa Field Guide is now available at OSU’s e-store (http://estore.osu-extension.org/productdetails.cfm?sku=827.) for purchase at $13/copy. (A 10% discount if more than 50 copies are ordered.)  The new version has a spiral binding to be able to flip through the pages more easily.  Many of the sections have new pictures and updates. In additions, new sections were added to address issues related to application technology and sampling and testing for mycotoxins. This book is a must have for the pickup truck and your sprayer as you scout corn, soybeans and wheat for weeds, insect, diseases and fertility issues. This guide puts many essential pieces of information at you finger tips in the field where you need it most.

Back-to-back Years of Vomitoxin Problems in Our CropsBack-to-back Years of Vomitoxin Problems in Our Crops

Back-to-back Years of Vomitoxin Problems in Our Crops

Gibberella ear rot and vomitoxin in corn 2009, head scab and vomitoxin in wheat in 2010, raise the question, should we be concerned about Gibberella ear rot and vomitoxin in corn in 2010?, if so, what can we do to minimize problems this year and next year and possibly the next year?

After last year’s corn crop with Gibberella ear rot and vomitoxin problems, wheat producers asked “what will happen to our wheat crop, given that both head scab and Gibberella ear rot are caused by the same fungus?” The answer was “it will depend on the weather and the level of scab resistance in your wheat variety”. Unfortunately, a very wet spring and early summer and susceptible wheat varieties gave us the answer we would have preferred not to hear. Now the question is again being asked “with this year’s scabby wheat, what does this mean for Gibberella ear rot in our corn crop?” Again, the answer is, it will depend on the weather and the Gibberella ear rot resistance of your corn hybrid.

No one can guarantee that if we did not have an ear rot problem in corn last year we would not have had the head scab problem in wheat this year. Similarly, no one can guarantee that because we have had a head scab problem in wheat this year we will also have a Gibberella ear rot problem in corn. Our last big scab epidemic was in 1996 and that epidemic did not come after or lead to a major Gibberella ear rot epidemic in corn. However, what we do know is that the risk of having a scab problem after a moldy corn crop or a Gibberella ear rot problem after a scabby wheat crop is much higher when weather conditions are favorable for infection than if both crops are healthy in both years.

A scabby wheat crop and a moldy corn crop mean that there will be more spores of Fusarium graminearum (the Gibberella ear rot, head scab and vomitoxin fungus) being carried over from one crop to another, especially since no-till or minimum tillage are a very standard part of our production system. The more crop residue on the soil surface, the more spores will survive to infect the next crop. If spores are already present and abundant and we plant susceptible varieties/hybrids, then all we’ll need to have a disease and vomitoxin problem is favorable weather occurring at the right time.

What can we do to minimize vomitoxin problems in our corn and wheat crops? The first approach would be to plant the best resistant variety/hybrid that you can find. For wheat we have Malabar and Bromfield, two varieties with resistance comparable to Truman and yield comparable to Hopewell. For corn, talk to you seed dealers; there are Gibberella ear rot resistant hybrids out there.

For wheat head scab, Triazoles fungicides such as Prosaro, Proline and Caramba are the best we have, and they do a very good job suppressing vomitoxin, especially when applied exactly at anthesis and used in combination with the best resistance. Use the scab forecasting system as a guide when using fungicides.

For Gibberella ear rot in corn, there are no such fungicide recommendations. This is because timing becomes a major issue when thinking about using a fungicide to manage Gibberella ear rot. Here is why, the fungus can infect the ear early during silk emergence, during grain fill, or even through the base of the ear late in the season. So, timing of fungicide applications if difficult when conditions may be favorable for infection at multiple times during the growing season.

The third and probably the most reliable approach for minimizing vomitoxin in our crops would be to destroy or plow under crop residue, especially after a scabby wheat crop or a moldy corn crop.

Corn Disease July Status Report

Most of our early-planted corn is now tasseling or will be within the next 7 to 10 days. Most of our important foliar diseases usually begin to develop at about this time, however, so far we have seen very little on the lower leaves. As usual, there are a few lesions of anthracnose and eye spot on the leaves close to the ground, but no gray leaf spot, northern corn leaf blight or any other foliar diseases have been reported statewide to date. These diseases usually develop best under wet, humid conditions. It has been very hot and dry in most areas, and the forecast is for more dry weather over the next few days. This will keep foliar diseases in check. However, keep monitoring the crop as it goes through pollination and early grain development to determine what is out there, look for:

Anthracnose - This is one of the first foliar diseases to develop on the young corn plant. Lesions are oval to elongate, with brownish borders. A yellowish area usually surrounds the diseased portions of the leaf. Anthracnose if favored by frequent rainfall and warm temperatures.

Anthracnose

Eye Spot- Small circular to oval lesions, with a tan to grayish center and a yellowish halo, beginning on the leaves below the ear and progressing up the plant. This disease tends to be very common in no-till fields, and is favored my moderate temperatures and abundant rainfall.

Eyespot

Gray leaf spot - Gray, rectangular lesions that develop first on the leaves below the ear. This disease usually begins developing close to or after tasseling and is favored by warm, humid conditions during the months of July and August. Like eye spot, gray leaf spot also tends to be more of a concern in no-till corn fields.

Gray Leaf Spot

Northern Corn Leaf Blight - This is another residue-borne disease that develops best under wet, humid conditions. However, it prefers slightly cooler conditions than those favorable for the development of gray leaf spot. Lesions are large, cigar-shaped and gray-green to tan in color.

Northern Corn Leaf Blight

Common Rust - This is not a residue-borne disease, as such, its development is not affected by crop rotation and tillage. Spores are blown in from the south. Like northern corn leaf blight, it develops best under cool, humid conditions. The symptoms are reddish-brown, oblong pustules scattered over both surfaces of the leaf.

Common Leaf Rust Corn

More information on all these diseases can be found at http://www.oardc.ohio-state.edu/ohiofieldcropdisease/corn/corn2.htm

Several fungicides are effective against foliar diseases of corn. These tend to be most profitable and the yield response most consistent when conditions are favorable for disease development and susceptible hybrids are planted, especially in a no-till, continuous-corn field. For more on foliar disease of corn and foliar fungicide, visit the field crops diseases web site and access the foliar fungicide efficacy table can be found at http://www.oardc.ohio-state.edu/ohiofieldcropdisease/corn/Corn%20foliar%20fungicide%20chart%202008.pdf

Western Bean Cutworm Counts Continue to Increase

The past week did not indicate any decrease in the number of western bean cutworm moths caught in Ohio.  In the past 7 days, 388 moths were caught, bringing our total to 405.  For perspective, we did not reach 405 moths until after July 27th in 2009 (and we only caught 151 total in 2008!).  Recent scouting trips have not yet detected the presence of any eggs, although most of the corn in Ohio has still not tasseled.  Any non-tasseled corn remains the preferred site for oviposition, therefore we have a lot of corn that is still at risk for western bean cutworm infestation.  As we head towards the middle of July, and as numbers begin to increase, scouting will become more important to determine if treatment will be needed.  Insecticides are only effective after larvae emerge and before they enter the ear, so proper identification and monitoring of egg masses will give you the best information for timing the application if needed.  Again, if you suspect the presence of egg masses, please contact your local extension educator or one of the field crop insect specialists (michel.70@osu.edu, 330-263-3730 or hammond.5@osu.edu, 330-263-3727).

Certified Crop Advisor Pre-Exam Training

A Certified Crop Adviser (CCA) Exam Training Session, sponsored by OSU Extension Champaign, Logan and Hancock Counties and the OSU Extension Agronomic Crops Team will be offered at the Hancock County Extension Office, 7868 County Road 140, Findlay, Ohio 45840 on July 20 - 21, 2010 beginning at 9:00 a.m. on the 20th and to adjourn by 4:00 p.m. on the 21st.  

This training session is designed to help participants understand the principles necessary to become a certified crop advisor and to assist in preparation for the state and international CCA exams.  It is not a crash course designed to cover all specific information necessary to pass the CCA exam.  However, it will cover some of the performance objectives and will assist students by giving better direction for independent study.

Registration is on a first-come, first-serve basis.  The fee is $195.00 per person, which covers the cost of instruction, lunches, handouts and other costs associated with the course over the two days.  Pre-registration by July 14th is requested for meal planning and for handouts to be ordered and/or printed in sufficient quantities.  Registrations after the July 14th deadline will be subject to materials on hand and we cannot guarantee handout availability.  

To register complete the registration form and make payment payable to OSU Extension – Logan County and send to

Wesley Haun
OSU EXtension
120 E. Sandusky Ave.  Suite #1

Bellefontaine OH 43311. 

For more information please contact Wes Haun at 937-599-4227, haun.17@cfaes.osu.edu or Harold Watters at 937-484-1526, watters.35@cfaes.osu.edu.
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About the C.O.R.N. Newsletter

C.O.R.N. Newsletter is a summary of crop observations, related information, and appropriate recommendations for Ohio crop producers and industry. C.O.R.N. Newsletter is produced by the Ohio State University Extension Agronomy Team, state specialists at The Ohio State University and the Ohio Agricultural Research and Development Center (OARDC). C.O.R.N. Newsletter questions are directed to Extension and OARDC state specialists and associates at Ohio State.